(1) Field of the Invention
This invention concerns linear electronic amplification systems, and relates in particular to methods and apparatus for improving the linearity of such systems incorporating distributed amplifiers.
(2) Description of the Prior Art
For many applications it is desirable that electronic amplifiers have good linearity--that is, that over the range concerned their output is a constant multiple of their input. A typical such application is in the field of high frequency radio transmission, where a wideband amplifier of the distributed-amplifier type feeds radio frequency power to a transmitting aerial for radiation. Various arrangements and designs have been employed to ensure either that the desired linearity is inherently attained or that any deviation from linearity is corrected by circuitry additional to the amplifier itself.
Some types of amplifier, however, have proven rather difficult in this respect: such a type is the distributed amplifier.
A distributed amplifier may be defined as an amplifier in which a number of individual amplifying members are arranged in parallel to take a control signal from one common input line and to feed an amplified signal to one common output line, the arrangement being such that the control signal arrives sequentially at each amplifying member, and such that all the individual outputs add in phase to give the maximum output. Each amplifying member thus contributes a part of the final output, which final output is effectively merely the sum of the individual parts. Distributed amplifiers are well known, and various types used as wideband amplifiers are described in, for example, our British Patent Specification No. 846,633 (corresponding to U.S. Pat. No. 3,129,387) and, more recently, in the Specification of our copending U.S. Application Ser. No. 88,063 filed Oct. 24th 1979 (I/6251/M).
A basic distributed amplifier format is described hereinafter with reference to the schematic circuit diagram of FIG. 1 of the accompanying drawings.
Distributed amplifiers are of particular value in the power amplification of high frequency signals such as those used to generate carrier waves in radio and television broadcasting. However, the correction of amplification errors in such amplifiers using tranditional techniques has proven particularly difficult because the relatively large time delay (and thus signal phase difference) "across" the amplifier has not easily allowed the precise control of both amplitude and phase at every stage that is required for good cancellation of amplifier-induced distortion. The present invention seeks to provide a distributed amplifier system allowing the easier correction of amplifier errors.